Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: makjoh on December 03, 2023, 10:00:14 am
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I'm trying to find the fault with my amp ( Laney 20 watt 3 x 12 ax7 and 2 EL 84) and I suspect it's the output transformer. The primary has red blue and black wires. Red to Black has 105 ohms, Blue to black 116 ohms. Red to Blue 221 ohms
Secondary has brown red and black wires. Brown to black is 0.5 ohms, red to black is 0.2 ohms same as red to brown.
Blue primary to red secondary is 5.8 mega ohms and Red primary to red secondary is 2.8 mega ohms
Primary to ground is 5.3 mega ohms and secondary to ground is 0.5 ohms
Am I correct in assuming the transformer is faulty because of readings from secondary to primary and to earth?
Many thanks for any experts advice on this
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none of the reading jump out as a problem ohms wise. Got a scope?
when playing does the OT get overly warm/hot?
do you have a thread on what the problem is n what you've done? if so a link might be helpful
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none of the reading jump out as a problem ohms wise. Got a scope?
when playing does the OT get overly warm/hot?
do you have a thread on what the problem is n what you've done? if so a link might be helpful
There is an original thread from a couple of years back. I gave up and put the amp aside until now. I'll see if its still there
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https://el34world.com/Forum/index.php?topic=27326.msg300125#msg300125
This is the original post
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PCB amps can be tough to diagnose. I recall (only after re-reading!) your post from 2021.
To clarify, you did mention that the tubes had been tested "good" in another amp, right? Meaning that when you used them in the other working amp, the amp worked . . .
I'd suggest taking hi-resolution photos of the circuit boards and the wiring, and post them. If you have trouble with posting images, see Doug's page @ https://el34world.com/Forum/index.php?board=2.0 (https://el34world.com/Forum/index.php?board=2.0)
Maybe we can see the problem.
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The primary has red blue and black wires.
Red to Black has 105 ohms, Blue to black 116 ohms.
Red to Blue 221 ohms
These seem like good readings for a transformer primary.
Secondary has brown red and black wires.
Brown to black is 0.5 ohms,
red to black is 0.2 ohms same as red to brown.
These seem like good readings for an output transformer secondary. It is few-turns of fat-wire, and resistance will be close to the resistance of your meter's leads.
Blue primary to red secondary is 5.8 mega ohms
Red primary to red secondary is 2.8 mega ohms
Primary to ground is 5.3 mega ohms
secondary to ground is 0.5 ohms
These all look like good readings:
- Primary is high-resistance to secondary.
- Primary is high-resistance to ground.
- Secondary is low-resistance to ground because one side is connected to ground.
- This winding's low resistance will fool one into believing all was are shorted to ground; these readings are normal.
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You can also do a poor man’s Pri:Sec VAC ratio test using the heater winding on your PT as an AC source* for the OT secondary (and read the resulting VAC across the OT primaries). If doing this, disconnect the PT heater winding from all other circuits first (and disconnect all the OT windings altogether- and take care to not let any loose winding ends touch each other, or the chassis, or yourself, or other components).
The lowish (I.e. 50-60Hz) voltage frequency won’t give a totally accurate ratio reading for the purpose of working out reactance etc, but it’s close enough for vac comparisons/confirmation that the thing is working correctly.
The OT Pri:Sec impedance ratio will be the square of the Pri:Sec VAC ratio.
*I keep an old 60W PT from a transistor guitar amp for this purpose - run the 50VAC secondary winding across a 25W rheostat to get the right source voltage- also doubles as a short prevention mechanism (if there’s a short in the OT)
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Looks like your transformer is likely ok.
Remember when measuring primary to secondary with a dc meter, dc won't pass through the transformer. Also, measuring from your transformer to ground, you may be measuring through capacitors which can't pass dc either.
What you haven't explained is the symptoms your amp displays to make you think the transformer is gone.
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*I keep an old 60W PT from a transistor guitar amp for this purpose - run the 50VAC secondary winding across a 25W rheostat to get the right source voltage- also doubles as a short prevention mechanism (if there’s a short in the OT)
@tubeswell,
If I apply Ohm's Law to using a 25W rheostat with a 50VAC source, I get a current of 0.5A. In selecting the rheostat, how did you determine in advance what the current load might be? I'm sure you threw in a safety factor, but how did you arrive at your estimate of the maximum power rating? Apologies if this is too off-topic.
Thanks!
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I suspected the output transformer was at fault because of the Megaohm readings on the multimeter and readings to earth. I was always led to believe they should be OL?
Anyway I've switched the amp on to assess it again. I can only see the one 12 ax7 glowing (next to the EL 84's)
Every other valve is stone cold :BangHead: These valves test good in a Laney VC 30 so I know they're fine. I'll start fault finding again and update
Thanks gents
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...fault because of the Megaohm readings on the multimeter and readings to earth. I was always led to believe they should be OL?
On old insulation, especially long-stored (not recently cooked-out), Megs is low but not necessarily bad.
> I can only see the one 12 ax7 glowing ....
So you have bad heater wires, a popular problem. The fault is usually between the last one which lights and the first dark tube.
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*I keep an old 60W PT from a transistor guitar amp for this purpose - run the 50VAC secondary winding across a 25W rheostat to get the right source voltage- also doubles as a short prevention mechanism (if there’s a short in the OT)
@tubeswell,
If I apply Ohm's Law to using a 25W rheostat with a 50VAC source, I get a current of 0.5A. In selecting the rheostat, how did you determine in advance what the current load might be? I'm sure you threw in a safety factor, but how did you arrive at your estimate of the maximum power rating? Apologies if this is too off-topic.
Thanks!
Not that scientific, I just went for the biggest rheostat that could handily fit into the project box that I mounted the PT onto - turned out to be a 25W 1k rotary jobbie. 50VAC/1,000R = 0.05A and 0.05 x 50 = 2.5W, so I can afford to have the pot rotated to 90% and everything will still be 'okay' if there is a short from the wiper to the return. (Worst case the OT is shorted and the bit between the rheostat input and the rheostat wiper takes up the heat). Good enough to do the job.
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...fault because of the Megaohm readings on the multimeter and readings to earth. I was always led to believe they should be OL?
On old insulation, especially long-stored (not recently cooked-out), Megs is low but not necessarily bad.
> I can only see the one 12 ax7 glowing ....
So you have bad heater wires, a popular problem. The fault is usually between the last one which lights and the first dark tube.
Thanks.......I've never known a problem like that. I'll get the meter on everything and report back soon
Thanks gents
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Not that scientific, I just went for the biggest rheostat that could handily fit into the project box...Good enough to do the job.
@tubeswell,
Still working on using and applying Ohm's Law correctly and your explanation helped a lot. Thanks again.
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I've taken some readings.The heater wires are 9.8 volts and 7.3 volts. This continues consistently throughout the board at all valve sockets.
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I've taken some readings.The heater wires are 9.8 volts and 7.3 volts. This continues consistently throughout the board at all valve sockets.
I think you are checking filament voltage wrong. To correctly measure filament you must measure with both meter leads connected directly to the socket. For EL84s one probe on pin 4 and the other probe on pin 5. For ECC83s one probe on pin4/5 and the other probe on pin 9.
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I've taken some readings.The heater wires are 9.8 volts and 7.3 volts. This continues consistently throughout the board at all valve sockets.
I think you are checking filament voltage wrong. To correctly measure filament you must measure with both meter leads connected directly to the socket. For EL84s one probe on pin 4 and the other probe on pin 5. For ECC83s one probe on pin4/5 and the other probe on pin 9.
Ahhhhhh......right, I wasn't aware of that method. I had the ground connected to the chassis and was measuring on the different solder blobs on the board where the sockets are connected
Thanks Sluckey
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All voltage readings check out fine. Is it possible for valves that sound fine in one amplifier (my Laney VC 30) can sound awful in a different amplifier (my Laney TT 20)? As is the case here, I'm testing the valves that sound good in the VC 30 but sound terrible in the TT 20, massive amounts of humming, hardly audible e.t.c.